Computer models are being developed to simulate the behavior of macromolecules involved in association equilibria in the ultracentrifuge and in gel chromatography, and the models are being applied to study association reactions of several proteins. Two general types of simulation programs are being used: distorted-grid models and finite-element procedures. Distorted-grid models have been written for single solutes and concentration-dependent mixtures, for rapid self-associations and for rapid mixed associations. The structure of existing finite-element programs has been improved to allow more efficient treatment of rapid self-associations; models are under development for rapid mixed associations and for slowly relaxing reactions. The distorted-grid approach is considerably more efficient for problems it can deal with than is the finite element model. The finite-element procedure has the advantage in treating slow reactions and reflected diffusion at the meniscus. The systems being examined by comparing simulated and real experiments include: (1) hemoglobin from the bullfrog tadpole, which dissociates to dimers and monomers in a pH dependent reaction, (2) chicken erythrocyte histones which undergo mixed association which is dependent on pH and salt concentration, and (3) components of nitrogenases from various bacterial sources.